Reactions happen only when particles collide with enough energy
Definition
Kinetic energy
The energy that an object or particle has because it is moving.
- Collision theory says that particles must collide with each other before they can react.
- A collision leads to a reaction only if the particles have enough kinetic energy.
- A collision that leads to a reaction is called a successful collision.
- The rate depends on how often the particles collide and on how many of those collisions are successful.
Key Idea
- Particles must collide, and collide hard enough, to react.
- More frequent or more energetic collisions give a faster reaction.
Activation energy is the least energy a collision needs to react
Definition
Activation energy
The minimum energy that reacting particles must have for a collision to result in a reaction.
- The activation energy is the minimum energy that colliding particles must have to react.
- If the particles collide with less than this energy, they simply bounce apart.
- Only collisions with at least the activation energy change the reactants into products.
Each factor speeds a reaction up by changing collisions
Definition
Concentration
The mass of solute present per unit volume of solution, usually measured in g/dm3\text{g}/\text{dm}^3g/dm3 for this calculation.
Definition
Surface area to volume ratio
The surface area of an object divided by its volume.
- Concentration and pressure: more particles in the same volume means collisions happen more often.
- Surface area: smaller solid pieces have a larger surface area to volume ratio, so more particles are exposed and collide.
- Temperature: the particles move faster, so they collide more often.
- A higher temperature also means more particles have at least the activation energy, so more collisions are successful.
- A catalyst: it lowers the activation energy, so a larger fraction of collisions are successful.
Common Mistake
- Do not say that heating simply gives particles more collisions; explain both effects.
- A higher temperature increases the collision frequency and the fraction of successful collisions.
Exam technique
- Answer a rate question in a chain: state the factor, then the effect on collisions, then the effect on rate.
- For temperature, always give both the more-frequent and the more-successful collisions.
- Use the words collision frequency and activation energy to gain the explanation marks.
Self review
- State the two conditions a collision needs to cause a reaction.
- What is meant by activation energy?
- Explain, using collisions, why a higher concentration speeds up a reaction.
- Why does a higher temperature have two effects on the rate?
